The present invention relates generally to imaging systems. More particularly, the present invention relates to x-ray tube cathodes with enhanced thermal performance.
Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. In an x-ray tube, for example, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube, and is removed from the vacuum vessel by a cooling fluid circulating over the exterior surface of the vacuum vessel. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses which are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary.
The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. A typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies to accelerate the electrons. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel.
In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports.
Since the production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. To cool the x-ray tube, the thermal energy generated during tube operation must be transferred from the anode through the vacuum vessel and be removed by a cooling fluid. The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other x-ray system or structure. Also, the casing is lined with lead to provide stray radiation shielding.
The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bipolar configuration. The performance of the cooling fluid may be degraded, however, by excessively high temperatures that cause the fluid to boil at the interface between the fluid and the vacuum vessel and/or the transmissive window. The boiling fluid may produce bubbles within the fluid that may allow high voltage arcing across the fluid, thus degrading the insulating ability of the fluid. Further, the bubbles may lead to image artifacts, resulting in low quality images. Thus, the current method of relying on the cooling fluid to transfer heat out of the x-ray tube may not be sufficient.
As X-ray tubes continue to grow in heat storage capability, the duration of an X-ray scan increases and the cooling time between scans decreases. The longer scans and shorter cool times require that the filaments in the cathode be held at high temperatures for a greater percentage of time. As a result, the cup that holds the filaments experiences higher temperatures than that of prior x-ray tubes.
In current high performance CT tubes, it has been observed that these higher temperatures can result in braze failures and distortions in the cathode arm. This results in image quality degradation. A conventional approach to the problem is to make a more conductive thermal path from the cathode cup to the cooler oil that lies in the X-ray tube. However, adding greater thermal conduction typically results in higher mass in the cathode support structure, while only marginally improving thermal performance. The higher mass often results in cathode vibration problems which compromise the x-ray tube's image quality.
Thus, there is a need for an apparatus which significantly increases the heat flow away from the cathode cup, resulting in cooler cathode assembly temperatures. Further, there is a need for a cathode design with greater ability to produce long duration scans without sacrificing image quality or long term reliability of the X-ray tube due to joint failure or mechanical component distortions. Even further, there is a need for a cathode design which greatly increases the heat flow from the cathode cup without producing a lower natural frequency in the cathode design due to added mass, resulting in good image quality while still giving good thermal performance of the cathode assembly.